Cell fate determination at specific stages of development is determined in part by quantitative differences in cell signaling. In such cases, cells with distinct differentiated phenotypes will differ from one another in terms of how much certain genes have been induced in their progenitors in response to the differential signaling. This project concerns the development of a genetic approach that is intended to inform on these sorts of differences. Novel reporter alleles have been generated that adopt different states as a function of how much Cre recombinase cells express. When used with appropriate Cre alleles, these new alleles are designed to inform on how differentiated populations of cells might differ with respect to how much prior gene expression they experienced. The reporter alleles are therefore of potential benefit for determining the basis of lineage commitment and for assessing whether quantitative differences in cell signaling might impact cell differentiation. Embryonic stem (ES) cells carrying the reporter alleles have been generated by gene targeting. In this project the ES cells will be used to create mice, and the behavior of the new alleles in them will be characterized in detail. This characterization will depend on crossing the new mice to other mice that express the Cre recombinase in an inducible or cell-type-specific fashion. Leukocytes will be used as a representative population, and a series of experiments will be conducted to determine if the recombination status of the reporter alleles can be directly correlated with Cre recombinase activity levels. The characterization of the new reporter alleles is essential for their potential value as investigative tools to be established, and to determine the extent to which they will be appropriate for informing on the gene expression history of cell populations. The alleles are intended to extend the range of tools available to investigators for use with the expanding range of Cre-expressing mouse strains that are under development in numerous laboratories. PUBLIC HEALTH RELEVANCE: This project is directed towards the development of improved methods for determining how cells differentiate during embryogenesis and immune responses. Imperfect cell differentiation can result in abnormal embryo development, poor immunity to infections, and cancer. Stem cell and other therapies are designed to restore, correct or redirect cell differentiation. The experiments proposed in this proposal could lead to a better understanding of how differentiation is controlled and how it might be manipulated for therapeutic benefit.